1. Field of the Invention
The present invention relates to a process for forming dielectric films, particularly to a process for forming dielectric films suitable for use as high permittivity gate dielectric films in a semiconductor device.
2. Description of the Related Art
In semiconductor devices like MOS (metal oxide semiconductor) type transistors, channel lengths are made shorter and shorter to increase the operation speed. However, the shortening of the channel length lowers the capacitance of the gate dielectric films and retards the switching operation of the transistors. Therefore to obtain a sufficient capacitance for the switching operation of the transistors, the gate dielectric films are made thinner. Conventionally as the material for the dielectric films of MOS type transistors, silicon dioxide (SiO2) is used owing to ease of the production and the resulting satisfactory interfacial properties of the film. However, the decrease of the thickness of the gate dielectric films to several nanometers tends to cause a larger amount of gate leakage current to increase the power consumption disadvantageously. One solution to this problem is formation of the gate dielectric films from a material having a relative permittivity constant higher than that of SiO2 (∈r=3.9). The dielectric films composed of such a material are called high permittivity dielectric films (high-k dielectric films). The high-k dielectric films having a higher permittivity constant can be made thicker for obtaining the capacitance comparative to that of the silicon dioxide film without the increase of the leakage current.
The material for the high-k dielectric films include metal dioxide such as ZrO2 and HfO2. A known method for depositing such a metal dioxide on a surface of a substrate is organometal chemical vapor deposition (MOCVD) as described in Japanese Patent Application Laid-Open No. 2004-140292 (US counterpart U.S. Pat. No. 7,105,362, US Application Publication No. 2006/0008969). In the MOCVD, a metal complex material is liquefied by heating in a vessel, vaporized by introducing a carrier gas into the vessel, and carried to a reaction chamber. The material carried to the reaction chamber deposits to form a film on the surface of a heated substrate.
However, the film formed by MOCVD contains a large amount of residual impurities like carbon and hydrogen coming from the starting organic material. The residual impurities tend to cause a large quantity of a leakage current in the formed film. Another known method is sputtering which enables deposition of a metal oxide containing a less amount of impurities unlike the MOCVD. In sputtering, high-energetic particles are allowed to collide against a surface of a metal target to repel the target-constituting atoms and to deposit the atoms on the surface. For example, an inert rare gas like argon is ionized by electric discharge by use of a target metal as a cathode and the formed ions are allowed to collide against the target metals to cause sputtering. Thereby a metal film containing less residual impurities can be deposited.
A conventional process for forming high-k dielectric films by sputtering is described below with reference to FIGS. 4A to 4D. In FIGS. 4A to 4D, the numerals denote followings: 401, a substrate composed of monocrystalline silicon or a like material; 402, a silicon dioxide film; 403, a metal film; 404, a metal silicate film; 405, a metal dioxide film.
In the step of FIG. 4A, the surface of substrate 401 is cleaned by RCA cleaning or a like cleaning method to eliminate contaminants from the surface to expose silicon atoms on the surface. Then, in the step of FIG. 4B, the surface portion of substrate 401 is oxidized to form silicon dioxide film 402. The oxidation of the surface portion of substrate 401 is conducted by thermal oxidation, radical oxidation, or a like method. In the next step shown in FIG. 4C, metal film 403 is deposited by sputtering on the surface of silicon dioxide film 402. In the next step shown in FIG. 4D, metal film 403 is oxidized by oxygen radicals to form a metal dioxide film 405. Since a crystallized metal dioxide film tends to cause leakage current in comparison with an amorphous metal dioxide film, the oxidation of metal film 403 is conducted with oxygen radicals, which enables oxidation at a lower temperature without crystallization during the oxidation. The oxygen radicals are generated by plasma excitation, photo-excitation, or a like method.
During the radical oxidation, a portion of the metal atoms constituting metal film 403 diffuses into silicon dioxide film 402 to form metal silicate film 404. Metal silicate film 404 is thermally more stable and less liable to crystallize than metal dioxide film 405.
Therefore, in order to obtain gate dielectric films having excellent thermal stability and reliability, preferably the mixing of metal film 403 and the silicon dioxide film is promoted to convert the metal dioxide in the film to silicate as much as possible.
On the other hand, Japanese Patent Application Laid-Open No. 2002-314074 (US counterpart U.S. Pat. No. 6,734,069□ US Application Publication No. 2003/0092238) discloses thermal oxidation at a high temperature for promoting diffusion of metal atoms. Japanese Patent Application Laid-Open No. 2003-297814 (US counterpart US Application Publication No. 2003/0185980) discloses irradiation of energetic particles for promoting diffusion of metal atoms.
In the above method of Japanese Patent Application Laid-Open No. 2004-140292, metal film 403 is oxidized at a low temperature, so that the metal atoms do not diffuse well. A longer time of oxidation by this method to diffuse the metal atoms enough into the silicon dioxide film can cause excessive oxidation. On the other hand, in the method of Japanese Patent Application Laid-Open No. 2002-314074, in which thermal oxidation is conducted at a high temperature to diffuse the metal atoms in a short time, the underlying silicon substrate tends to be oxidized to increase excessively the thickness of the silicon dioxide film.
In the method of Japanese Patent Application Laid-Open No. 2003-297814, in which the metal atoms are diffused only by irradiation of energetic particles, the thickness of the remaining silicon dioxide film cannot readily be controlled.
As described above, in conventional techniques, the diffusion of the metal into the silicon dioxide film and the mixing of the silicon dioxide film with the metal film cannot readily be controlled, and the conditions of diffusion of the metal atoms and the conditions of the oxidation of metal film cannot be controlled independently. Thus conventional techniques cannot produce dielectric films having a desired film thickness with a high productivity. Measures to solve the problems are demanded.